The history of drought in eastern Australia

A project undertaken at The University of Newcastle and supervised by Russell Drysdale

Drought is the most costly natural hazard facing Australia. It occurs at least once a decade, yet predicting its onset and likely magnitude remains elusive for climate forecasters. A major reason for this is a lack of climate records that cover periods of time of sufficient length to encompass the full spectrum of climate variability. In Australia, most instrumental records cover only the last 100-130 years. Whilst records of this length help our understanding of El Niño-La Niña, they are too short to come to grips with longer-phase phenomena, such as the multi-decadal Pacific Decadal Oscillation. To extend climate records beyond the period of instrumental measurements we must resort to natural archives that are responsive to climate variability. Cave stalagmites are used in many parts of the world to investigate past variations in climate, but few such studies have been undertaken in Australia. This project will use geochemical information preserved in stalagmites from Wombeyan Caves to reconstruct the history of drought in eastern Australia. Stalagmites are highly responsive to climate variability because the composition of the drip waters that feed them changes with shifts in regional temperatures, biological activity and rainfall. In shallow caves, stalagmites are particularly responsive to rainfall variations, and, if they grow fast enough, they can preserve a record of rainfall history at annual resolution. A recent pilot study carried out at Wombeyan Caves revealed that the chemical composition of drip waters in shallow cave chambers responded systematically and unambiguously to the dry conditions of the 2002-2003 El Nino. These ‘drought signals’ are embedded within the calcite of a growing stalagmite. In this study, two active stalagmites will be collected from Wombeyan and laser-sampled at very high resolution (10-20 microns) for these geochemical variations that act as a proxy for rainfall. The aim is to reconstruct the history of El Niño and la Niña over the last 1000 years so that the magnitude and frequency of historical events can be placed within a longer-term hydroclimatic context. A time slice between 6000 and 8000 years ago will also be examined to evaluate El-Niño – La Niña behaviour during the warmest part of the present interglacial, which some believe is a potential analogue for climate change over the next 50-100 years.
Research team
This research is led by Dr Russell Drysdale, a geoscientist and karst specialist from the University of Newcastle. Russell is carrying out similar projects in central Italy, Northern Territory, Christmas Island and other parts of NSW. Hermon Slade funds will be used to cover costs of analytical work, which includes precision radiometric dating by Dr John Hellstrom and laser-ablation trace element analyses by Dr Roland Maas, both at the University of Melbourne. Janece McDonald, whose PhD research (near completion) stimulated this project, will work on the project as a postdoctoral student. The team is working closely with the Sydney Catchment Authority, who are charged with supplying water to Sydney and the greater metropolitan area. Data from this study will assist, amongst others, hydrologists in better understanding long-term rainfall variability. The research team gratefully acknowledges the support of the Hermon Slade Foundation and the Jenolan Caves Reserve Trust. 
Figure 1. Environmental conditions above a cave are ultimately recorded in stalagmites.

Figure 2. Active stalagmites such as these have the potential to preserve detailed records of climate history

Figure 3. Drip monitoring apparatus used by Janece McDonald at Wombeyan to understand the links between climate, cave drip water chemistry and stalagmite geochemistry. Results of Janece’s work provided the background to this research